Engine control method and engine system

ABSTRACT

When an incremental amount of a steering angle exceeds a reference incremental amount, an ECU  60  executes vehicle attitude control of reducing an output torque of an engine, and, in a given operating range, drives a spark plug  16  in a manner allowing an air-fuel mixture to be self-ignited at a given timing, thereby executing SPCCI combustion. When there is a request for an additional deceleration from the vehicle attitude control (# 12 : YES), and the SPCCI combustion is performed (# 13 : YES), the ECU  60  executes fuel amount reduction control of reducing the amount of fuel to be supplied into a cylinder  2  (# 14 ), so as to attain torque reduction for the vehicle attitude control. On the other hand, when the SPCCI combustion is not performed (# 13 : NO), the ECU  60  executes ignition retardation control of retarding an ignition timing of the spark plug  16  (# 15 ).

TECHNICAL FIELD

The present invention relates to a control method for an engineconfigured such that a part of an air-fuel mixture is combusted in sparkignition (SI) combustion, and the remainder is combusted byself-ignition, i.e., in compression ignition (CI) combustion, whereinthe engine is capable of changing its output torque (torque to begenerated) according to a steering angle, and an engine system using thecontrol method.

BACKGROUND ART

There has been known premixed charge compression ignition combustion (orhomogenous charge compression ignition combustion) in which a mixture ofair and gasoline fuel is sufficiently compressed in a cylinder so as tobe combusted by self-ignition. Further, there has been proposedpartially premixed charge compression ignition combustion (hereinafterreferred to as “spark controlled compression ignition (SPCCI)combustion” which is a combination of spark ignition (SI) combustion andcompression ignition (CI) combustion, instead of combusting the entiretyof an air-fuel mixture by self-ignition (see, for example, the followingPatent Document 1). In the SPCCI combustion, a part of an air-fuelmixture is forcibly combusted by flame propagation triggered by sparkignition (SI combustion), and then the remaining unburned air-fuelmixture is combusted by self-ignition (CI combustion).

Meanwhile, there has also be known driving support control ofcomprehensively controlling accelerations (G) in a forward-rearward(longitudinal) direction and a width (lateral) direction of a vehicle bychanging an output torque of an engine according to a steering angle(this control will hereinafter be referred to as “vehicle attitudecontrol”). In the vehicle attitude control, immediately after a driverstarts to turn a steering wheel, the output torque of the engine isreduced to be less than a required torque, to generate a deceleration Gin the vehicle, thereby causing a load shift toward front road wheels.This results in increases in tire gripping force and cornering force ofthe front road wheels. In the vehicle attitude control, the engineoutput torque is reduced by means of, e.g., retardation of an ignitiontiming at which an air-fuel mixture is ignited by a spark plug (ignitionretardation).

CITATION LIST Patent Document

Patent Document 1: JP 2001-073775A

Patent Document 2: JP 6112304B

SUMMARY OF INVENTION Technical Problem

There is a demand to execute the vehicle attitude control in a vehiclemounting an engine capable of the SPCCI combustion. However, if, in asituation where the SPCCI combustion is performed, the engine outputtorque is reduced by the ignition retardation so as to execute thevehicle attitude control, an in-cylinder pressure is likely to fail torise up to a value required for the CI combustion in a late phase of theSPCCI combustion, resulting in the occurrence of misfire.

It is an object of the present invention to provide an engine controlmethod capable of executing the vehicle attitude control withoutexerting an influence on combustion performance of the SPCCI combustion,and an engine system using the engine control method.

Solution to Technical Problem

According to one aspect of the present invention, there is provided acontrol method for an engine which is mounted to a vehicle havingsteerable road wheels and mechanically coupled to drive road wheels ofthe vehicle, and which includes a spark plug and a fuel injector, thecontrol method comprising: a combustion mode setting step of selecting acombustion mode of the engine between a first combustion mode in whichan entirety of an air-fuel mixture in a cylinder of the engine iscombusted by a propagation of a flame produced by the spark plug, and asecond combustion mode in which at least a part of an air-fuel mixturein the cylinder is combusted by a self-ignition, on the basis of anoperating state of the engine; a decremental torque setting step ofsetting a torque reduction amount by which an output torque of theengine is to be reduced, on the basis of a steering angle of thesteerable road wheels; a first torque reduction step of controlling thespark plug based on the torque reduction amount set in the decrementaltorque setting step so as to retard an ignition timing, when the firstcombustion mode is selected in the combustion mode setting step; and asecond torque reduction step of controlling the fuel injector based onthe torque reduction amount set in the decremental torque setting stepso as to reduce a fuel injection amount, when the second combustion modeis selected in the combustion mode setting step.

According to another aspect of the present invention, there is providedan engine system, comprising: an engine mounted to a vehicle havingsteerable road wheels and mechanically coupled to drive road wheels ofthe vehicle, the engine including a spark plug and a fuel injector; anoperating state sensor configured to detect an operating state of theengine; a steering angle sensor configured to detect a steering angle ofthe steerable road wheels; and a controller, wherein the controller isconfigured to: select a combustion mode of the engine between a firstcombustion mode in which an entirety of an air-fuel mixture in acylinder of the engine is combusted by a propagation of a flame producedby the spark plug, and a second combustion mode in which at least a partof an air-fuel mixture in the cylinder is combusted by a self-ignition,on the basis of a detection result by the operating state sensor; set atorque reduction amount by which an output torque of the engine is to bereduced, on the basis of a detection result by the steering anglesensor; control the spark plug based on the set torque reduction amountso as to retard an ignition timing, when the first combustion mode isselected as the combustion mode of the engine; and control the fuelinjector based on the set torque reduction amount so as to reduce a fuelinjection amount, when the second combustion mode is selected as thecombustion mode of the engine.

In the control method or the engine system of the present invention, thetorque reduction amount is set based on the steering angle of thesteerable road wheels. This operation is equivalent to execution of thevehicle attitude control. Further, the first combustion mode isequivalent to the SI combustion, and the second combustion mode isequivalent to the SPCCI combustion. Then, when the first combustion modeis selected as the combustion mode, the output torque of the engine isreduced by the set torque reduction amount, by means of retardation ofthe ignition timing (first torque reduction step). This operation isequivalent to torque reduction by ignition retardation.

On the other hand, when the second combustion mode is selected as thecombustion mode, the engine output torque is reduced by the set torquereduction amount, by means of reduction of the fuel injection amount(second torque reduction step). Specifically, during the SPCCIcombustion, the vehicle attitude control is executed by means of thefuel amount reduction control, instead of the ignition retardation.Thus, the start timing of the SI combustion in the SPCCI combustion isnot retarded. Therefore, an in-cylinder temperature and an in-cylinderpressure are sufficiently raised by heat generated by the SI combustion,so that it is possible to generate the CI combustion in the late phaseof the SPCCI combustion in a good manner without causing the occurrenceof misfire.

Preferably, the control method of the present invention furthercomprises an air/fuel ratio mode setting step of, when the secondcombustion mode is selected in the combustion mode setting step,selecting an air/fuel ratio mode between a first air/fuel ratio mode inwhich the air-fuel mixture is set to be leaner than a stoichiometricair/fuel ratio, and a second air/fuel ratio mode in which the air-fuelmixture is set to be equal to or richer than the stoichiometric air/fuelratio, on the basis of the operating state of the engine, wherein thesecond torque reduction step includes controlling the fuel injectorbased on the torque reduction amount set in the decremental torquesetting step so as to reduce the fuel injection amount, when the firstair/fuel ratio mode is selected in the air/fuel ratio mode setting step.

If the ignition retardation is performed during the first air/fuel mode,the lean air-fuel mixture causes difficulty in inducing self-ignition,so that the possibility of misfire becomes higher. According to theabove feature, when the vehicle attitude control is executed in asituation where the SPCCI combustion is performed in the first air/fuelratio mode, the second torque reduction step of reducing the fuelinjection amount is performed, so that it is possible to effectivelysuppress misfire.

Preferably, the control method of the present invention furthercomprises: an air/fuel ratio mode setting step of, when the secondcombustion mode is selected in the combustion mode setting step,selecting an air/fuel ratio mode between a first air/fuel ratio mode inwhich the air-fuel mixture is set to be leaner than a stoichiometricair/fuel ratio, and a second air/fuel ratio mode in which the air-fuelmixture is set to be equal to or richer than the stoichiometric air/fuelratio, on the basis of the operating state of the engine; and a thirdtorque reduction step of, when the second air/fuel ratio mode isselected in the air/fuel ratio mode setting step, controlling the sparkplug based on the torque reduction amount set in the decremental torquesetting step so as to retard the ignition timing.

When the air-fuel mixture is lean, the possibility of misfire becomeshigher, and, on the other hand, in the second air/fuel ratio mode inwhich the air-fuel mixture is formed to have an air/fuel ratio equal toor less than the stoichiometric air/fuel ratio, the possibility ofmisfire becomes relatively low even if the ignition retardation isperformed. According to the above feature, when the vehicle attitudecontrol is executed in a situation where the SPCCI combustion isperformed in the second air/fuel ratio mode, the second torque resectionstep based on the ignition retardation is performed. That is, thevehicle attitude control can be executed by control of drive timing(ignition timing) of the spark plug which is relatively simple control.

Effect of Invention

The present invention can provide an engine control method capable ofexecuting the vehicle attitude control without exerting an influence oncombustion performance of the SPCCI combustion, and an engine systemusing the engine control method.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic diagram of a vehicle using an engine controlmethod and an engine system according to one embodiment of the presentinvention.

FIG. 2 is a system diagram showing the entire configuration of acompression ignition engine using the engine control method according tothis embodiment.

FIG. 3 is a block diagram showing a control system of the compressionignition engine.

FIG. 4 is an operating range map for explaining different combustioncontrols according to an engine speed and an engine load.

FIG. 5 illustrates time charts for schematically explaining combustioncontrols to be executed in respective ranges of the operating range mapin FIG. 4.

FIG. 6 is a graph showing a heat release rate during execution of sparkcontrolled compression ignition (SPCCI) combustion.

FIG. 7 illustrates time charts schematically showing a control stateduring vehicle attitude control.

FIG. 8 is a flowchart showing a specific example of the vehicle attitudecontrol.

FIG. 9 is a graph showing a relationship between a steering speed and atarget additional deceleration.

FIGS. 10(A) and 10(B) are flowcharts schematically showing the enginecontrol method according to this embodiment.

FIG. 11 is a flowchart showing a basic operation of an engine controlmethod according to this embodiment.

FIG. 12 is a flowchart showing the details of an engine controlprocessing subroutine in the engine control method according to thisembodiment.

FIG. 13 is a flowchart showing the details of the engine controlprocessing subroutine.

FIG. 14 is a flowchart showing the details of the engine controlprocessing subroutine.

FIG. 15 is a flowchart showing the details of the engine controlprocessing subroutine.

FIG. 16 is a tabular diagram showing a relationship between a total fuelinjection amount and an ignition timing, in each of various operatingmodes.

FIG. 17 illustrates time charts showing the state of mode switchingbetween a first air/fuel ratio mode (λ>1) and a second air/fuel ratiomode (λ=1).

DESCRIPTION OF EMBODIMENTS

[Structure of Vehicle]

An embodiments of the present invention will now be described in detailbased on the drawings. First of all, with reference to FIG. 1, thestructure of a vehicle 100 using an engine control method and an enginesystem according to one embodiment of the present invention will beschematically described. The vehicle 100 pertaining to this embodimentis a front-engine, front-wheel drive (FF) vehicle, and is equipped withan engine 1 as a driving source. The engine 1 is an in-linefour-cylinder gasoline engine which has four cylinders 2 and is capableof spark ignition (SI) combustion and spark controlled compressionignition (SPCCI) combustion.

The vehicle 100 comprises: a vehicle body 101 mounting the engine 1; twofront road wheels 102 serving as drive road wheels and steerable roadwheels; and two rear road wheels 103 serving as driven road wheels. Adriving force generated by the engine 1 is transmitted to the front roadwheels 102 via a transmission 104. The vehicle 100 is also equipped witha steering wheel 105 for steering the front wheels 102, and a powersteering unit 106 for assisting manipulation of the steering wheel 105.Further, the vehicle 100 is equipped with an accelerator pedal 107configured to be manipulated by a driver and to adjust the degree ofopening of the after-mentioned throttle valve 32.

The vehicle 100 is equipped with an ECU 60 (controller) forelectronically controlling the engine 1. The ECU 60 in this embodimentis configured to be capable of executing vehicle attitude control whenthe driver manipulates the steering wheel 105. In the vehicle attitudecontrol, immediately after the driver starts to turn the steering wheel105, an output torque to be generated from the engine 1 is reduced to beless than a required torque determined by a depression amount (relativeposition) of the accelerator pedal 107 or the like, to generate adeceleration G in the vehicle 100, thereby causing a load shift towardthe front road wheels 102. This results in increases in tire grippingforce and cornering force of the front road wheels 102. The vehicleattitude control and the SPCCI combustion will be described in detaillater.

[Engine System]

Next, the engine system equipped in the vehicle 100 will be described.FIG. 2 is a diagram showing the entire configuration of the enginesystem according to this embodiment. This engine system comprises theengine 1 composed of a four-stroke direct gasoline-injection engine,wherein the engine 1 comprises: an engine body; an intake passage 30 forallowing intake air to flow therethrough so as to be introduced into theengine body, an exhaust passage 40 for allowing exhaust gas to flowtherethrough so as to be discharged from the engine body, and an EGRdevice 50 for allowing the exhaust gas flowing through the exhaustpassage 40 to be partly recirculated to the intake passage 30.

The engine 1 is used as a driving source of the vehicle 100. In thisembodiment, the engine 1 is of a type to be driven by receiving supplyof fuel consisting mainly of gasoline. Here, the fuel may be gasolinecontaining bioethanol or the like. The engine 1 comprises a cylinderblock 3, a cylinder head 4, and four pistons 5. The cylinder block 3 hasfour cylinder liners each forming therein a respective one of thecylinders. The cylinder head 4 is attached to an upper surface of thecylinder block 3 to close upper openings of the cylinders 2. Each of thepistons 5 is received in a respective one of the cylinders 2 in areciprocatingly slidable manner, and coupled to a crankshaft 7 via aconnecting rod 8. The crankshaft 7 is configured to be rotated about acentral axis thereof according to reciprocating movement of the pistons5.

A combustion chamber 6 is defined above each of the pistons 5. The abovefuel is injected and supplied from the after-mentioned injector 15 intothe combustion chamber 6. Then, a mixture of air and the supplied fuelis combusted in the combustion chamber 6, so that the piston 5 pusheddownwardly by an expansive force of the combustion will bereciprocatingly moved in an upward-downward direction. A geometriccompression ratio of the cylinder 2, i.e., a ratio of the volume of thecombustion chamber 6 as measured when the piston 5 is at a top deadcenter position to the volume of the combustion chamber 6 as measuredwhen the piston 5 is at a bottom dead center position, is set to a highcompression ratio of 13 to 30 (e.g., about 20) so as to become suited tothe SPCCI combustion.

The cylinder block 3 is installed with a crank angle sensor SN1 and awater temperature sensor SN2. The crank angle sensor SN1 is configuredto detect a rotational angle of the crankshaft 7 (crank angle), and arotational speed of the crankshaft 7 (engine speed). The watertemperature sensor SN2 is configured to detect the temperature ofcooling water flowing through the cylinder block 3 and the cylinder head4 (engine water temperature).

In each of the cylinders 2, the cylinder head 4 is formed with an intakeport 9 and an exhaust port 10 each communicated with the combustionchamber 6. A bottom surface of the cylinder head 4 serves as a ceilingsurface of the combustion chamber 6. This ceiling surface of thecombustion chamber is formed with an intake-side opening which is adownstream end of the intake port 9, and an exhaust-side opening whichis an upstream end of the exhaust port 10. Further, an intake valve 11for opening and closing the intake-side opening and an exhaust valve 12for opening and closing the exhaust-side opening are assembled to thecylinder head 4. Although illustration is omitted, a valve arrangementof the engine 1 is an intake-side two-valve×exhaust-side two-valve,four-valve type in which each of the intake port 9 and the exhaust port10 is provided by a number of two (one pair) per cylinder 2, and each ofthe intake value 11 and the exhaust valve 12 is also provided by anumber of two (one pair) per cylinder 2.

The cylinder head 4 is provided with an intake-side valve operatingmechanism 13 and an exhaust-side valve operating mechanism 14 eachcomprising a camshaft. Each of the pair of intake valves 11 and the pairof exhaust valves 12 is configured to be driven by a corresponding oneof the valve operating mechanisms 13, 14 in an openable and closeablemanner, interlockingly with the rotation of the crankshaft 7. Theintake-side valve operating mechanism 13 has a built-in intake-sidevariable valve timing mechanism (intake VVT) 13 a capable of changing atleast a valve opening timing of the pair of intake valves 11. Similarly,the exhaust-side valve operating mechanism 14 has a built-inexhaust-side variable valve timing mechanism (exhaust VVT) 14 a capableof changing at least a valve closing timing of the pair of exhaustvalves 12. By controlling the intake VVT 13 a and the exhaust VVT 14 a,it is possible to adjust a valve overlap period during which both thepair of intake valves 11 and the pair of exhaust valves 12 aremaintained in an open state across top dead center of an exhaust stroke.Further, by adjusting the valve overlap period, it is possible to adjustthe amount of burned gas (internal EGR gas) remaining in the combustionchamber 6.

In each of the cylinders 2, the cylinder head 4 is further provided withan injector 15 (fuel injector) and a spark plug 16. The injector 15 isconfigured to inject (supply) fuel into the cylinder 2 (combustionchamber 6). As the injector 15, it is possible to use a multi-holeinjector capable of injecting fuel in a radial pattern from a pluralityof nozzle holes formed at a distal end thereof. The injector 15 isdisposed such that the distal end thereof is exposed to the inside ofthe combustion chamber 6, and opposed to a radially central region of acrown surface of the piston 5.

The spark plug 16 is disposed at a position slightly offset toward theintake side with respect to the injector 15, and a distal end(electrode) thereof is disposed at a position facing the inside of thecylinder 2. The spark plug 16 is a forced ignition source for ignitingan air-fuel mixture formed in the cylinder 2 (combustion chamber 6).

The cylinder head 4 is installed with an in-cylinder pressure sensorSN3, an intake cam angle sensor SN12 and an exhaust cam angle sensorSN13 each serving as a sensing element. The in-cylinder pressure sensorSN3 is configured to detect an internal pressure of the combustionchamber 6 in each of the cylinders 2 (in-cylinder pressure). The intakecam angle sensor SN12 is configured to detect a rotational position ofthe camshaft (cam angle) of the intake-side valve operating mechanism13, and the exhaust cam angle sensor SN13 is configured to detect arotational position of the camshaft (cam angle) of the exhaust-sidevalve operating mechanism 14.

As shown in FIG. 2, the intake passage 30 is connected to one lateralsurface of the cylinder head 4, in such a manner as to be communicatedwith the pair of intake ports 9 in each of the cylinders 2. Air (fleshair) taken from an upstream end of the intake passage 30 is introducedinto the combustion chamber 6 through the intake passage 30 and the pairof intake ports 9. The intake passage 30 is provided with an air cleaner31, a throttle valve 32, a supercharger 33, an electromagnetic clutch34, an intercooler 35, and a surge tank 36, which are arranged in thisorder from the upstream end thereof.

The air cleaner 31 is configured to remove foreign substances containedin intake air, thereby cleaning the intake air. The throttle valve 32 isconfigured to open and close the intake passage 30, interlockingly witha depressing movement of the accelerator pedal 107, thereby adjustingthe flow rate of intake air in the intake passage 30. The supercharger33 is configured to compress intake air and send the compressed intakeair toward a downstream end of the intake passage 30. The supercharger33 is a mechanical supercharger mechanically coupled to the engine 1,and configured such that engagement with the engine 1 and release of theengagement are switched by the electromagnetic clutch 34. When theelectromagnetic clutch 34 is engaged, a driving force is transmittedfrom the engine 1 to the supercharger 33, to drive the supercharger 33to perform supercharging. The intercooler 35 is configured to cool theintake air compressed by the supercharger 33. The surge tank 36 is atank disposed immediately upstream of a non-illustrated intake manifoldto provide a space for equally distributing intake air to the fourcylinders 2.

The intake passage 30 is provided with: an air flow sensor SN4 to detectthe flow rate of intake air; first and second intake temperature sensorsSN5, SN7 to detect the temperature of intake air; first and secondintake pressure sensors SN6, SN8 to detect the pressure of intake air,in respective regions thereof. The air flow sensor SN4 and the firstintake temperature sensor SN5 are disposed in a region between the aircleaner 31 and the throttle valve 32 in the intake passage 30 to detectthe flow rate and the temperature of intake air passing through theregion, respectively. The first intake pressure sensor SN6 is disposedin a region between the throttle valve 32 and the supercharger 33(downstream of a connection with a downstream end of the after-mentionedEGR passage 51) in the intake passage 30, to detect the pressure ofintake air passing through the region. The second intake temperaturesensor SN7 is disposed in a region between the supercharger 33 and theintercooler 35 in the intake passage 30 to detect the temperature ofintake air passing through the region. The second intake pressure sensorSN8 is disposed in the surge tank 36 to detect the pressure of intakeair inside the surge tank 36.

The intake passage 30 includes a bypass passage 38 for sending intakeair to the combustion chambers 6 while bypassing the supercharger 33.The bypass passage 38 mutually connects the surge tank 36 and thevicinity of the downstream end of the after-mentioned EGR passage 51.The bypass passage 38 is provided with a bypass valve 39 capable ofselectively opening and closing the bypass passage 38.

The exhaust passage 40 is connected to the other lateral surface of thecylinder head 4, in such a manner as to be communicated with the pair ofexhaust ports 10 in each of the cylinders 2. Burned gas (exhaust gas)produced in the combustion chamber 6 is discharged to the outside of thevehicle 100 through the pair of exhaust ports 10 and the exhaust passage40. The exhaust passage 40 is provided with a catalytic converter 41.The catalytic converter 41 houses a three-way catalyst 41 a forpurifying harmful components (HC, CO, NOx) contained in exhaust gasflowing through the exhaust passage 40, and a gasoline particulatefilter (GPF) 41 b for capturing particulate matter (PM) contained in theexhaust gas.

The EGR device 50 comprises: an EGR passage 51 connecting the exhaustpassage 40 and the intake passage 30; and an EGR cooler 52 and an EGRvalve 53 each provided in the EGR passage 51. The EGR passage 51mutually connects a region of the exhaust passage 40 located downstreamof the catalytic converter 41 and a region of the intake passage 30located between the throttle valve 32 and the supercharger 33. The EGRcooler 52 is configured to cool exhaust gas (external EGR gas) which isbeing recirculated from the exhaust passage 40 to the intake passage 30through the EGR passage 51, in a heat-exchanging manner. The EGR valve53 is provided in the EGR passage 51 in a selectively openable andclosable manner at a position downstream of the EGR cooler 52 to adjustthe flow rate of exhaust gas flowing through the EGR passage 51. The EGRpassage 51 is installed with a pressure difference sensor SN9 to detecta difference between the pressure of the external EGR gas at a positionupstream of the EGR valve 53 and the pressure of the external EGR gas ata position downstream of the EGR valve 53.

The accelerator pedal 107 is provided with an accelerator positionsensor SN10 to detect the relative position of the accelerator pedal 107(accelerator positon) (to serve as one operating state sensor).Specifically, the accelerator position sensor SN10 is a sensor to detectthe degree of depression of the accelerator pedal 107, and also a sensorto detect driver's acceleration/deceleration manipulation. The steeringwheel 105 is associated with a steering angle sensor SN11. The steeringangle sensor SN11 is configured to detect a steering angle of the frontwheels 102 based on a rotation angle of the steering wheel 105. Itshould be understood that it is possible to use any other type ofsteering angle sensor capable of detecting a steering angle of the frontwheels 102.

[Control Configuration]

FIG. 3 is a block diagram showing a control configuration of the enginesystem. The engine system according to this embodiment iscomprehensively controlled by the ECU (engine control module) 60. TheECU 60 is a microprocessor which comprises a CPU, a ROM and a RAM.

The ECU 60 is configured to accept an input of detection signals fromvarious sensors installed in the vehicle 100. The ECU 60 is electricallyconnected with the crank angle sensor SN1, the water temperature sensorSN2, the in-cylinder pressure sensor SN3, the air flow sensor SN4, thefirst and second intake temperature sensors SN5, SN7, the first andsecond intake pressure sensors SN6, SN8, the pressure difference sensorSN9, the accelerator position sensor SN10, the steering angle sensorSN11, the intake cam angle sensor SN12 and the exhaust cam angle sensorSN13. Plural pieces of information detected by these sensors SN1 toSN13, i.e., information such as the crank angle, the engine speed, theengine water temperature, the in-cylinder pressure, the intake air flowrate, the intake air temperature, the intake air pressure, the pressuredifference before and after the EGR valve 53, the accelerator position,the steering angle and the intake and exhaust cam angles, aresequentially input to the ECU 60.

The ECU 60 is operable to control each part of the engine whileexecuting various determinations and computations based on input signalsfrom the sensors SN1 to SN13 and others. Specifically, the ECU 60 iselectrically connected to the intake VVT 13 a, the exhaust VVT 14 a, theinjector 15, the spark plug 16, the throttle valve 32, theelectromagnetic clutch 34, and the bypass valve 39, the EGR valve 53,and others, and is operable, based on results of the computations, etc.,to output control signals, respectively, to these devices.

The ECU 60 functionally comprises a combustion control part 61, avehicle attitude control part 62 and a determination part 63. Thecombustion control part 61 is operable to control a fuel injectionoperation of the injector 15, and an ignition operation of the sparkplug 16. For example, the combustion control part 61 is operable, basedon the engine speed detected by the crank angle sensor SN1, an engineload (required torque) identified from the relative position of theaccelerator pedal 107 detected by the accelerator position sensor SN10,and the intake air flow rate detected by the air flow sensor SN4, todetermine a fuel injection amount and a fuel injection timing of theinjector 15, and an ignition timing of the spark plug 16, and drive theinjector 15 and the spark plug 16 in accordance with the resultingdetermined values. In this process, the combustion control part 61operates to refer to a predetermined operating range map (one examplethereof is shown in FIG. 4) to select a combustion mode. Although thedetails will be described later, the combustion mode includes acombustion mode in which the injector 15 and the spark plug 16 aredriven to cause an air-fuel mixture in each cylinder 2 to beself-ignited at a given timing (SPCCI combustion).

The vehicle attitude control part 62 is operable to execute the vehicleattitude control configured to change the output torque of the engine 1according to the steering angle of the front road wheels 102 responsiveto manipulation of the steering wheel 105. For example, the vehicleattitude control part 62 is operable to refer to a detection value ofthe steering angle sensor SN11, and, when the steering angle isincreased by a given amount or more within a given time period, todetermine that the vehicle 100 is in a turning (cornering) state, andexecute control of reducing the output torque. In this embodiment, meansfor the torque reduction may include, but is not particularly limitedto, retardation control of retarding the ignition (drive) timing of thespark plug 16 (first torque reduction step), and fuel amount reductioncontrol of reducing the amount of fuel to be supplied into each cylinder2 (second torque reduction step), which are selectively employedaccording to operating modes or the like. Here, the vehicle attitudecontrol part 62 is operable to execute each of the controls such that,as a torque reduction amount required in the vehicle attitude controlbecomes larger, the ignition timing is more largely retarded, or thefuel injection amount is more largely reduced.

The determination part 63 is operable to determine whether or notcombustion in the combustion chamber 6 is likely to come into a state inwhich it is unstable or there is a possibility of misfire (combustionunstable state). In this embodiment, combustion control (includingcontrol of the SPCCI combustion) by the combustion control part 61 andthe vehicle attitude control by the vehicle attitude control part 62 areoverlappingly executed. If the two controls are overlappingly executedunder certain conditions, the combustion unstable state can be caused.Further, the determination part 63 is operable, when the combustionstate is determined to be likely to come into the combustion unstablestate, to execute control of changing a control mode of the combustioncontrol or the vehicle attitude control.

[Combustion Control]

Next, the combustion control to be executed by the combustion controlpart 61 will be described in detail. FIG. 4 is a simplified operatingrange map for explaining different combustion controls according to anengine speed and an engine load. This operating range map shows fouroperating ranges: a first range A1; a second range A2; a third range A3;and a fourth range A4. The first range A1 is a combination of a range inwhich the engine speed is in a low and intermediate region, and theengine load is in a low (including no load) region, and a range in whichthe engine speed is in a high region, and the engine load is in anintermediate and high region. The second range A2 is a range (low andintermediate speed-intermediate load range) in which the engine speed isin the low and intermediate region, and the engine load is in a regionhigher than that of the first range A1. The third range A3 is a range(low and intermediate speed-high load range) in which the engine speedis in the low and intermediate region, and the engine load is in aregion higher than that of the second range A2. The fourth range A4 is arange in which the engine speed is in the low region, and the engineload is close to a full-load line.

In the first range A1 and the fourth range A4, the SI combustion (firstcombustion mode) is performed. The SI combustion is a combustion type inwhich an air-fuel mixture in the combustion chamber 6 is ignited byspark ignition using the spark plug 16, and forcibly combusted by flamepropagation causing expansion of a combustion region from the ignitionpoint toward surroundings thereof. That is, the SI combustion is acombustion mode in which the entire air-fuel mixture in each cylinder 2is combusted by propagation of flame produced by the spark plug 16.

In the second range A2 and the third range A3, the SPCCI combustion(second combustion mode) is performed. The SPCCI combustion is acombination of the SI combustion and the CI combustion. The CIcombustion is a combustion type in which an air-fuel mixture iscombusted by self-ignition in an environment where the air-fuel mixtureis highly raised in temperature and pressure according to compression bythe piston 5. The SPCCI combustion is a combustion type in which a partof an air-fuel mixture in the combustion chamber 6 is subjected to theSI combustion by spark ignition performed in an environment close tothat causing self-ignition of the air-fuel mixture, and, after the SIcombustion, the remaining air-fuel mixture in the combustion chamber 6is subjected to the CI combustion by self-ignition (caused by highertemperature and pressure resulting from the SI combustion). That is, theSPCCI combustion is a combustion mode in which at least a part of anair-fuel mixture in each cylinder 2 is combusted by self-ignition.

In this embodiment, there are two air/fuel ratio modes: a first air/fuelratio mode (λ>1) in which an air-fuel mixture formed in the combustionchamber 6 for the SPCCI combustion is set to be leaner than astoichiometric air/fuel ratio; and a second air/fuel ratio mode (λ≤1) inwhich the air-fuel mixture is set to be equal to or richer than thestoichiometric air/fuel ratio. More specifically, the first air/fuelratio mode is a mode in which the SPCCI combustion is performed while anair/fuel ratio (A/F) as a weight ratio of air (fresh air) to fuel in thecombustion chamber 6 is set to a value greater than the stoichiometricair/fuel ratio (14.7). On the other hand, the second air/fuel ratio modeis a mode in which the SPCCI combustion is performed while the air/fuelratio (A/F) is set to be equal to the stoichiometric air/fuel ratio(λ=1) or slightly less than the stoichiometric air/fuel ratio (λ<1). Inthis embodiment, the air/fuel ratio A/F of an air-fuel mixture to beformed in the first air/fuel ratio mode is set in the range of about 2to 30/1. Further, the air/fuel ratio A/F in the second air/fuel ratio istypically λ=1 or 14.7/1. For the SPCCI combustion, either one of thefirst air/fuel ratio mode (λ>1) or the second air/fuel ratio mode (λ≤1)is selected based on the engine operating state (air/fuel ratio modesetting step).

FIG. 5 illustrates time charts for schematically explaining combustioncontrols to be executed in respective ranges A1 to A4 of the operatingrange map in FIG. 4. The chart (a) of FIG. 5 shows the fuel injectiontiming, the ignition timing and the combustion state (waveform of heatrelease rate) as measured when the engine is operated at an operatingpoint P1 included in the second range A2 illustrated in FIG. 4. In thesecond range A2, the SPCCI combustion is performed in the first air/fuelratio mode (λ>1).

The combustion control to be executed by the combustion control part 61at the operating point P1 is as follows. As shown in the chart (a), theinjector 15 is controlled to inject fuel during a period from anintermediate phase to a late phase of a compression stroke, in a mannerdivided into two stages: a 1st fuel injection; and a 2nd fuel injection.The spark plug 16 is controlled to ignite an air-fuel mixture at atiming adjacent to and on a slightly advance side with respect to topdead center of a compression stroke. This ignition triggers the start ofthe SPCCI combustion, so that a part of an air-fuel mixture in thecombustion chamber is combusted (subjected to the SI combustion) byflame propagation, and the remaining air-fuel mixture is combusted(subjected to the CI combustion) by self-ignition.

Here, with reference to FIG. 6, advantages of the SPCCI combustion willbe described. FIG. 6 is a graph showing the heat release rate duringexecution of the SPCCI combustion. The SPCCI combustion has a propertythat heat release increases more steeply when the CI combustion isdeveloped than when the SI combustion is developed. Specifically, asshown in FIG. 6, in the SPCCI combustion, a rising slope in an earlyphase corresponding to the SI combustion is gentler than a rising slopein the subsequent phase corresponding to the CI combustion. When the SIcombustion causes increases in internal temperature and pressure of thecombustion chamber 6, the remaining unburned air-fuel mixture isself-ignited to start the CI combustion. At this timing when the CIcombustion is started (the inflection point X in FIG. 6=crank angleθci), the slope of a waveform of the heat release rate changes fromgentle to steep. Further, in the SPCCI combustion, along with such atendency of the heat release rate, an increase rate (dp/dθ) of theinternal pressure of the combustion chamber 6 appearing during the SIcombustion is smaller than that appearing during the CI combustion.

After the start of the CI combustion, the SI combustion and the CIcombustion are performed in parallel. In terms of a combustion speed ofthe air-fuel mixture, the CI combustion is greater than the SIcombustion. Thus, the CI combustion exhibits a relatively large heatrelease rate. However, the slope of the waveform of this heat releaserate never becomes excessive, because the CI combustion is developedafter top dead center of a compression stroke. Specifically, afterpassing through top dead center of a compression stroke, a motoringpressure is lowered due to a downward movement of the piston 5, andthereby the rise of the heat release rate is suppressed, so that thesituation where the dp/dθ during the CI combustion becomes excessive isavoided. As above, in the SPCCI combustion, by its property that the CIcombustion is performed after the SI combustion, the dp/dθ serving as anindex of combustion noise is less likely to become excessive, so that itis possible to suppress combustion noise, as compared to simple CIcombustion (a case where the entire fuel is subjected to the CIcombustion).

When the CI combustion is completed, the SPCCI combustion is alsocompleted. The CI combustion is greater than the SI combustion in termsof the combustion speed. Thus, the SPCCI combustion is capable ofadvancing a combustion completion timing, as comparted to the simple CIcombustion (the case where the entire fuel is subjected to the CIcombustion). In other words, in the SPCCI combustion, it is possible toallow the combustion completion timing to come close to top dead centerof a compression stroke, in an expansion stroke. This makes it possibleto improve fuel economy performance in the SPCCI combustion, as comparedto the simple SI combustion.

Returning to FIG. 5, the chart (b) shows the state of combustion controlwhich is executed by the combustion control part 61 when the engine isoperated at an operating point P2 included in the third range A3illustrated in FIG. 4 (included in a relatively low load region of thethird range A3). In the low load region of the third range A3,combustion of an air-fuel mixture adjusted to λ=1, which fall into thesecond air/fuel ratio mode (λ≤1) in the SPCCI combustion, is performed.

At the operating point P2, the combustion control part 61 operates tocause the injector 15 to perform a first fuel injection for injectingfuel in an intake stroke in a relatively large amount, and subsequentlyperform a second fuel injection for injecting fuel in a compressionstroke in an amount less than that of the first fuel injection. Further,the combustion control part 61 operates to cause the spark plug 16 toignite an air-fuel mixture at a timing on a slightly advance side withrespect to top dead center of the compression stroke. This ignitiontriggers the start of the SPCCI combustion, in the same manner as thatat the operating point P1.

The chart (c) of FIG. 5 shows the state of combustion control which isexecuted by the combustion control part 61 when the engine is operatedat an operating point P3 included in the third range A3 (included in arelatively high load region of the third range A3). In the high loadregion of the third range A3, control of subjecting, to the SPCCIcombustion, an air-fuel mixture whose air/fuel ratio in the combustionchamber 6 is set to be slightly richer than the stoichiometric air/fuelratio (λ≤1) is executed.

At the operating point P3, the combustion control part 61 operates tocause the injector 15 to inject the entirety or most part of fuelrequired per combustion cycle, in an intake stroke. For example, fuel isinjected in a continuous period from a late phase of an intake stroke toan early phase of a compression stroke, as shown in the chart (c).Further, the combustion control part 61 operates to cause the spark plug16 to ignite an air-fuel mixture at a timing on a slightly retard sidewith respect to top dead center of the compression stroke. This ignitiontriggers the start of the SPCCI combustion, in the same manner as thatin the operating points P1, P2.

FIG. 5 shows one example in which an air-fuel mixture formed to have anair/fuel ratio equal to the stoichiometric air/fuel ratio (λ=1) and anair-fuel mixture formed to have an air/fuel ratio slightly richer thanthe stoichiometric air/fuel ratio (λ≤1) are selectively used dependingon the engine load. Alternatively, an air-fuel mixture may be formed tohave an air/fuel ratio equal to the stoichiometric air/fuel ratio (λ=1),in the entire third range A3. In the following description, thisembodiment will be described on the assumption that, in the secondair/fuel ratio mode to be performed in the third range A3, an air-fuelmixture having λ=1 is selected, and subjected to the SPCCI combustion.

The chart (d) of FIG. 5 shows the state of combustion control which isexecuted by the combustion control part 61 when the engine is operatedat an operating point P4 included in the fourth range A4 as a low speedand high load range. In the fourth range A4, SI combustion byretardation of ignition timing (retardation_SI) is performed, instead ofthe SPCCI combustion.

At the operating point P4, the combustion control part 61 operates tocause the injector 15 to perform a first fuel injection for injectingfuel in an intake stroke in a relatively large amount, and subsequentlyperform a second fuel injection for injecting fuel in a late phase of acompression stroke (immediately before top dead center of thecompression stroke) in an amount less than that of the first fuelinjection. Further, the combustion control part 61 operates to cause thespark plug 16 to perform ignition at a retarded timing. The ignitiontiming for an air-fuel mixture is set to a relatively largely retardedtiming after top dead center of the compression stroke by about 5 to 20°C.A. This ignition triggers the start of the SI combustion, and theentire air-fuel mixture in the combustion chamber 6 is combusted byflame propagation. Here, the reason that the ignition timing in thefourth range A4 is retarded in this manner is to prevent abnormalcombustion such as knocking or preignition.

The chart (e) of FIG. 5 shows the state of combustion control which isexecuted by the combustion control part 61 when the engine is operatedat an operating point P5 included in the high load and high speed regionof the first range A1. In the first range A1, normal SI combustion(intake_SI) is performed, instead of the SPCCI combustion.

At the operating point P5, the combustion control part 61 operates tocause the injector 15 to inject fuel in a continuous period from anintake stroke to a compression stroke. Here, the operating point P5 isin high load and high speed conditions. Thus, the amount of fuel to beinjected per combustion cycle is originally large, and a crank angleperiod necessary to inject a required amount of fuel is extended. On theother hand, in the intermediate and low load range, the fuel injectionamount is reduced, as compared to that in the chart (e). Further, thecombustion control part 61 operates to cause the spark plug 16 to ignitean air-fuel mixture at a timing on a slightly advance side with respectto top dead center of the compression stroke. This ignition triggers thestart of the SI combustion, and the entire air-fuel mixture in thecombustion chamber 6 is combusted by flame propagation.

[Vehicle Attitude Control]

Next, the vehicle attitude control to be executed by the vehicleattitude control part 62 will be described in detail. FIG. 7 illustratestime charts schematically showing a control state during vehicleattitude control in this embodiment. More specifically, FIG. 7 shows arelationship among the steering angle of the front road wheels 102responsive to manipulation of the steering wheel 105, the decelerationof the vehicle 100 by the vehicle attitude control, and the outputtorque required for realizing the deceleration.

When the amount of change in the steering angle of the steering wheel105 detected by the steering angle sensor SN11 becomes equal to orgreater than a reference amount (a steering speed becomes equal to orgreater than a given value), the vehicle attitude control part 62operates to deem it to be a situation where the vehicle 100 iscornering, and gradually increase the deceleration. As previouslydescribed, an output torque to be produced by the engine 1 is reduced bythe retardation control of retarding the ignition timing of the sparkplug 16 or the fuel amount reduction control of reducing the amount offuel to be supplied into each cylinder 2, thereby lowering the drivingforce of the vehicle 100 and increasing the deceleration of the vehicle100.

Specifically, the vehicle attitude control part 62 operates to reducethe engine output torque, with respect to a target basic engine torquewhich is a required engine torque during normal operation, anddetermined based on a vehicle speed detected by the crank angle sensorSN1 and the relative position of the accelerator pedal 107 detected bythe accelerator position sensor SN10. Then, when the steering speedbecomes less than the given value, the vehicle attitude control part 62operates to gradually reduce the deceleration. This makes it possible toincrease a corning force of the front road wheels 102 during cornering,thereby allowing the vehicle 100 to be smoothly turned.

With reference to a flowchart illustrated in FIG. 8, a specific exampleof the vehicle attitude control will be described. In FIG. 8,considering the meaning of adding a deceleration by means of torquereduction with respect to the target basic engine torque, the vehicleattitude control is referred to as “additional deceleration settingprocessing”. Upon start of an additional deceleration setting processingroutine, the vehicle attitude control part 62 operates to determinewhether or not the absolute value of the steering angle acquired from aresult of the detection by the steering angle sensor SN11 is increasing(step #1). When the absolute value of the steering angle is determinedto be increasing (YES in the step #1), the vehicle attitude control part62 operates to calculate the steering speed from the acquired steeringangle (step #2).

Subsequently, the vehicle attitude control part 62 operates to determinewhether or not the absolute value of the steering speed calculated inthe step #2 is decreasing (step #3). When the absolute value of thesteering speed is determined not to be decreasing (NO in the step #3),i.e., the absolute value of the steering speed is determined to beincreasing or the absolute value of the steering speed is determined notto be changing, the vehicle attitude control part 62 operates to set atarget additional deceleration based on the steering speed (step #4).This target additional deceleration is a deceleration to be added to thevehicle 100 according to manipulation of the steering wheel 105 intendedby a driver.

Specifically, the vehicle attitude control part 62 operates to acquire avalue of the target additional deceleration corresponding to thesteering speed calculated in the step #2, based on a relationshipbetween the target additional deceleration and the steering speed,represented by the map of FIG. 9. Referring to FIG. 9, when the steeringspeed is equal to or less than a given threshold T_(S), a correspondingvalue of the target additional deceleration is 0. That is, when thesteering speed is equal to or less than the threshold T_(S), the vehicleattitude control part 62 operates to avoid executing the control ofreducing the engine output torque so as to add a deceleration to thevehicle 100 (vehicle attitude control), even if the steering wheel 105is manually turned. On the other hand, when the steering speed isgreater than the threshold T_(S), a value of the target additionaldeceleration corresponding to this steering speed comes closer to agiven upper limit D_(max) (e.g., 1 m/s²). That is, as the steering speedbecomes larger, the target additional deceleration becomes larger, andthe increase rate of the target additional deceleration becomes smaller.

Subsequently, the vehicle attitude control part 62 operates to determinea maximum increase rate R_(max) which is a threshold of an additionaldeceleration to be used when the deceleration is added to the vehicle100. Then, the vehicle attitude control part 62 operates to determine anadditional deceleration in a current processing cycle (current-cycleadditional deceleration), under the condition that the increase rate ofthe current-cycle additional deceleration is equal to or less than themaximum increase rate R_(max) (step #5).

Specifically, when an increase rate from the additional decelerationdetermined in the last processing cycle (last-cycle additionaldeceleration) to the target additional deceleration set in the step #4in the current processing cycle is equal to or less than the maximumincrease rate R_(max), the vehicle attitude control part 62 operates todetermine the target additional deceleration set in the step #4, as thecurrent-cycle additional deceleration. On the other hand, when theincrease rate from the last-cycle additional deceleration to the targetadditional deceleration set in the step #4 in the current processingcycle is greater than the R_(max), the vehicle attitude control part 62operates to determine, as the current-cycle additional deceleration, avalue obtained by increasing the last-cycle additional deceleration atthe maximum increase rate R_(max).

Referring to the step #3 again, when the absolute value of the steeringspeed is determined to be decreasing (YES in the step #3), the vehicleattitude control part 62 operates to determine the last-cycle additionaldeceleration as the current-cycle additional deceleration (step #6).That is, when the absolute value of the steering speed is decreasing, anadditional deceleration corresponding to a maximum value of the steeringspeed (i.e., a maximum value of the additional deceleration) ismaintained.

Referring to the step #1 again, when the absolute value of the steeringangle is determined not to be increasing (NO in the step #1), thevehicle attitude control part 62 operates to set an amount (decelerationreduction amount) by which the last-cycle additional deceleration is tobe reduced in the current processing cycle (step #7). This decelerationreduction amount is calculated based on a constant reduction rate (e.g.,0.3 m/s³) preliminarily stored in a memory or the like comprised in theECU 60. Alternatively, the deceleration reduction amount may becalculated based on a reduction rate determined according to a drivingstate of the vehicle 100 obtained from various sensors or the steeringspeed calculated in step #2. Then, the vehicle attitude control part 62operates to determine the current-cycle additional deceleration bysubtracting the deceleration reduction amount set in the step #7 fromthe last-cycle additional deceleration (step #8).

Subsequently, the vehicle attitude control part 62 operates to determinea torque reduction amount, based on the current-cycle additionaldeceleration determined in the step #5, #6 or #8 (step #9: reductiontorque setting step). Specifically, the vehicle attitude control part 62operates to determine a value of the torque reduction amount requiredfor realizing the current-cycle additional deceleration, based oncurrent values of vehicle speed, road grade, a currently set one of aplurality of gear stages of a transmission, and others. Then, thevehicle attitude control part 62 operates to execute the retardationcontrol of retarding the ignition timing of the spark plug 16 or thefuel amount reduction control of reducing the amount of fuel to besupplied into each cylinder 2, through the combustion control part 61,so as to reduce the engine output torque by an amount corresponding tothe determined torque reduction amount.

[Control of Switching Among Plural Torque Reduction Means]

As mentioned above, in the engine system according to this embodiment,when the amount of change per unit time in the steering angle becomesequal to or greater than a preliminarily set reference value(hereinafter referred to as “satisfaction of a first condition”), thevehicle attitude control of reducing the output torque of the engine 1is executed. On the other hand, in the engine system according to thisembodiment, as the type of combustion of an air-fuel mixture in thecombustion chamber 6, not only the SI combustion (first combustion mode)but also the SPCCI combustion (second combustion mode) are performed.Specifically, when a required torque determined by the acceleratorposition and the vehicle speed falls into the second range A2 or thethird range A3 illustrated in FIG. 4 (hereinafter referred to as“satisfaction of a second condition”), the SPCCI combustion in which anair-fuel mixture is self-ignited at a given timing is performed. Eitherone of the SI combustion and the SPCCI combustion is selected accordingto the operating state of the engine (combustion mode setting step).

The vehicle attitude control part 62 operates to, when determiningsatisfaction of the first condition, execute the vehicle attitudecontrol (see FIG. 8). The vehicle attitude control part 62 also operatesto, when determining satisfaction of the second condition, control thefuel injection timing of the injector 15 and the drive (ignition) timingof the ignition plug 16 so as to develop the SPCCI combustion (see FIG.5). Further, in the SPCCI combustion, a mode switching will be performedbetween the first air/fuel ratio mode (λ>1) in which an air-fuel mixtureis formed to have an air/fuel ratio leaner than the stoichiometricair/fuel ratio, and the second air/fuel ratio mode (λ≤1) in which theair-fuel mixture is formed to have an air/fuel ratio equal to or richerthan the stoichiometric air/fuel ratio (see the charts (b), (c) of FIG.5).

Thus, when the first condition and the second condition aresimultaneously satisfied, the vehicle attitude control and the SPCCIcontrol will be overlappingly executed. That is, in a state in which theSPCCI combustion is performed, the reduction of the engine out put canbe performed to execute the vehicle attitude control. The retardation ofthe ignition timing of the spark plug 16 (ignition retardation) is thesimplest as means for the torque reduction. However, if, in the state inwhich the SPCCI combustion is performed, the ignition retardation isperformed to execute the vehicle attitude control, the combustion islikely to become unstable. Specifically, if the start timing of the SIcombustion in the SPCCI combustion is retarded due to the ignitionretardation, the in-cylinder pressure of the combustion chamber 6 islikely not to rise to a value required for the CI combustion in the latephase of the SPCCI combustion. In this situation, combustion in thecombustion chamber 6 is likely to come into a state in which it isunstable or there is a possibility of misfire (combustion unstablestate).

In view of the above, in this embodiment, the determination part 63operates to determine whether or not a current operating state of theengine is likely to lead to the combustion unstable state. Specifically,it is determined whether the first condition and the second conditionare simultaneously satisfied. Then, when the current operating state isdetermined to be likely to lead to the combustion unstable state, thedetermination part 63 operates to change the engine output torquereduction means for execution of the vehicle attitude control, from theignition retardation to the fuel amount reduction control of reducingthe amount of fuel to be supplied into each cylinder 2. When the fuelinjection amount is reduced beyond a value set with respect to arequired torque, the engine output torque is naturally reduced, withoutperforming the ignition retardation. Further, the timing of forcedignition to an air-fuel mixture by the spark plug 16 is maintained at atiming set for the SPCCI combustion, so that the SI combustion in theearly phase of the SPCCI combustion is started at a regular timing.Thus, it is possible to develop given SPCCI combustion.

The above torque reduction means switching control by the determinationpart 63 will be described with reference to the flowcharts illustratedin FIGS. 10(A) and 10(B). FIG. 10(A) shows one example in which theengine output torque reduction means for execution of the vehicleattitude control is switched, depending on whether or not the SPCCIcombustion is performed, i.e., the engine is operated in the secondrange A2 or the third range A3 in the operating range map of FIG. 4.

Upon start of an engine control processing routine, the ECU 60 (FIG. 3)operates to read various sensor signals regarding the driving state ofthe vehicle 100 (step #11). Specifically, the ECU 60 operates to acquirea variety of information including the vehicle speed obtained from adetection signal of the crank angle sensor SN1, the relative position ofthe accelerator pedal 107 detected by the accelerator position sensorSN10, the steering angle of the steering wheel 105 detected by thesteering angle sensor SN11, and a currently set one of the gear stagesof the transmission of the vehicle 100.

Subsequently, the determination part 63 operates to determine whether ornot there is a request for the additional deceleration, i.e., there is arequest for the torque reduction for execution of the vehicle attitudecontrol (whether or not the first condition is satisfied) (step #12).When the incremental amount of the steering angle exceeds a referenceincremental amount, the vehicle attitude control part 62 operates toissue a request for the additional deceleration (YES in the step #12).In this case, the determination part 63 operates to determine whether ornot the SPCCI combustion is performed by the combustion control part 61(whether or not the second condition is satisfied) (step #13). On theother hand, when there is no request for the additional deceleration (NOin the step #12), the determination part 63 operates to complete onecycle of the processing routine (return to the step #11).

When the SPCCI combustion is performed (YES in the step #13), thedetermination part 63 operates to cause the vehicle attitude controlpart 62 to execute the fuel amount reduction control of reducing thefuel injection amount of the injector 15, so as to perform the torquereduction for the vehicle attitude control (step #14). That is, when thefirst condition and the second condition are satisfied, thedetermination part 63 operates to cause the vehicle attitude controlpart 62 to execute the fuel amount reduction control of reducing theamount of fuel to be supplied into each cylinder 2, so as to attain thetorque reduction for the vehicle attitude control (second torquereduction step). Here, as the torque reduction amount becomes larger,the degree of reduction of the fuel injection amount is set to becomelarger.

On the other hand, when the SPCCI combustion is not performed (NO in thestep #13), i.e., when the engine is operated in the first range A1 orthe fourth range A4 in the operating range map of FIG. 4, thedetermination part 63 operates to cause the vehicle attitude controlpart 62 to execute the retardation control of retarding the ignitiontiming at which an air-fuel mixture is ignited by the spark plug 16, soas to perform the torque reduction for the vehicle attitude control(step #15). That is, when the first condition is satisfied but thesecond condition is not satisfied, the determination part 63 operates toretard the drive timing of the spark plug 16 so as to reduce the outputtorque of the engine 1 (first torque reduction step). Here, as arequired torque reduction amount becomes larger, the degree ofretardation of the ignition timing is set more largely. After executionof the steps #14 and #15, the determination part 63 operates to completeone cycle of the processing routine (return to the step #11).

As above, in the example illustrated in FIG. 10(A), the determinationpart 63 operates to, when determining satisfaction of the firstcondition and the second condition, reduce the engine output torque bymeans of the fuel amount reduction control, instead of the ignitionretardation. That is, during the SPCCI combustion, the vehicle attitudecontrol is executed by means of the fuel amount reduction control,instead of the ignition retardation. Thus, the start timing of the SIcombustion in the SPCCI combustion is not retarded. Therefore, thein-cylinder temperature and pressure are sufficiently raised by heargenerated by the SI combustion, so that it is possible to adequatelyproduce the CI combustion in the late phase of the SPCCI combustion,without causing misfire. On the other hand, when the SI combustion isperformed, instead of the SPCCI combustion, the misfire problem does notsubstantially occur. In this case, the vehicle attitude control isexecuted by means of the ignition retardation, so that it is possible tosimplify the control.

FIG. 10(B) shows another example in which the engine output torquereduction means for execution of the vehicle attitude control isswitched, depending on whether or not the SPCCI combustion is performed,and, when YES, the SPCCI combustion is performed in the first air/fuelratio mode (λ>1) using an air-fuel mixture with a lean air/fuel ratio,i.e., the engine is operated in the second range A2 in the operatingrange map of FIG. 4.

Processings in steps #21 and #22 are the same as those in the steps #11and #12 of the above example, and therefore description thereof will beomitted. When the vehicle attitude control part 62 is operating to issuethe request for the additional deceleration (YES in the step #22), thedetermination part 63 operates to determine whether or not thecombustion control part 61 is operating to perform the SPCCI combustionin the first air/fuel ratio mode (λ>1) (whether or not the secondcondition is satisfied, and, when YES, the first air/fuel ratio mode isperformed) (step #23).

When the SPCCI combustion in the first air/fuel ratio mode is performed(YES in the step #23), the determination part 63 operates to cause thevehicle attitude control part 62 to execute the fuel amount reductioncontrol of reducing the fuel injection amount of the injector 15, so asto perform the torque reduction for the vehicle attitude control (step#24). That is, when the first condition and the second condition aresatisfied, and the first air/fuel ratio mode (λ>1) is performed, thedetermination part 63 operates to cause the vehicle attitude controlpart 62 to execute the fuel amount reduction control of reducing theamount of fuel to be supplied into each cylinder 2 so as to reduce theoutput torque of the engine 1 (second torque reduction step).

On the other hand, when the SPCCI combustion in the first air/fuel ratiomode is not performed (NO in the step #23), i.e., when the engine isoperated in the first range A1 or the fourth range A4 in the operatingrange map of FIG. 4, the determination part 63 operates to cause thevehicle attitude control part 62 to execute the retardation control ofretarding the ignition timing at which an air-fuel mixture is ignited bythe spark plug 16, so as to perform the torque reduction for the vehicleattitude control (step #25). That is, when the first condition issatisfied but the second condition is not satisfied, or when the firstcondition and the second condition are satisfied, and the secondair/fuel ratio mode is performed, the determination part 63 operates toretard the drive timing of the spark plug 16 so as to reduce the outputtorque of the engine 1 (first or third torque reduction step).

As above, in the example illustrated in FIG. 10(B), the determinationpart 63 operates to, when determining satisfaction of the firstcondition and the second condition, and further determining that thefirst air/fuel ratio mode (λ>1) is performed, reduce the engine outputtorque by the fuel amount reduction control, instead of the ignitionretardation. That is, during the SPCCI combustion using an air-fuelmixture with a lean air/fuel ratio, the vehicle attitude control isexecuted by the fuel amount reduction control, instead of the ignitionretardation. If the ignition retardation is performed during combustionin the first air/fuel ratio mode (λ>1), the lean air-fuel mixture causesdifficulty in self-ignition, thereby increasing the probability ofmisfire. However, in this example, when the vehicle attitude control isexecuted in the situation where the SPCCI combustion in the firstair/fuel ratio mode (λ>1) is performed, the fuel amount reductioncontrol is executed, so that it is possible to effectively suppressmisfire.

When the air-fuel mixture is lean, the possibility of misfire becomeshigher, and, on the other hand, in the second air/fuel ratio mode inwhich the air-fuel mixture is formed to have an air/fuel ratio equal toor less than the stoichiometric air/fuel ratio, the possibility ofmisfire becomes relatively low even if the retardation control isexecuted. In this example, when the vehicle attitude control is executedin the situation where the SPCCI combustion in the second air/fuel ratiomode is performed, the torque reduction by the ignition retardation isemployed. Thus, the vehicle attitude control can be executed by controlof the drive timing (ignition timing) of the ignition plug 16, which isrelatively simple control.

[Specific Example of Engine Control Method]

Next, a specific example of operation control using the engine controlmethod according to this embodiment will be described. FIG. 11 is aflowchart showing a basic operation of the engine control methodaccording to this embodiment. Upon start of a processing routine, theECU 60 (FIG. 3) operates to read sensor signals regarding the drivingstate of the vehicle 100 output from the sensors SN1 to SN13 (step S1).Then, the ECU 60 (the vehicle attitude control part 62) operates torefer to the vehicle speed (crank angle sensor SN1), the acceleratorposition (accelerator position sensor SN10), the steering angle(steering angle sensor SN11), a currently set one of the gear stages ofthe transmission of the vehicle 100, etc., which are obtained from thesensor signals read in the step S1, to execute processing of setting theadditional deceleration (torque reduction amount) for the vehicleattitude control (step S2: decremental torque setting step). A specificexample of this additional deceleration setting processing subroutine isas previously described based on the flowchart of FIG. 8. Then, the ECU60 operates to execute an engine control processing subroutine, whiletaking into account the additional deceleration set in the step S2 (stepS3). With reference to flowcharts illustrated in FIGS. 12 to 15, theengine control processing subroutine in the step S3 will be described indetail.

<Setting of Combustion Process Control Target Values>

FIG. 12 is a flowchart showing the details of the engine controlprocessing subroutine, and mainly showing steps for setting combustioncontrol target values. Upon start of the control processing subroutine,the ECU 60 (combustion control part 61) operates to refer to the vehiclespeed, the accelerator position, the current gear stage, etc., acquiredin the step S1 illustrated in FIG. 11, to set a target acceleration(target G) of the vehicle 100 (step S11). Then, the ECU 60 operates toset a target basic engine torque necessary to realize the set targetacceleration (step S12). This target basic engine torque is a requiredtorque calculated based on the amount of depression of the acceleratorpedal 107 by the driver (accelerator position), i.e., a required torquebefore taking into account the torque reduction for the vehicle attitudecontrol.

Subsequently, the ECU 60 operates to set a target combustion mode, fromthe target basic engine torque, and a current value of the engine speeddetected by the crank angle sensor SN1 (step S13: combustion modesetting step). This target combustion mode is set by referring to, e.g.,the operating range map illustrated in FIG. 4, preliminarily defined bythe relationship between the engine speed and the engine load.Specifically, the ECU 60 operates to determine to which of the first tofourth ranges A1 to A4 in the operating range map the current value ofthe engine speed and the target basic engine torque (engine load) set inthe step S12 belong, and set, as the target combustion mode, acorresponding one of the combustion modes illustrated in the charts (a)to (e).

Subsequently, the ECU 60 (determination part 63) operates to set meansto attain the torque reduction for the vehicle attitude control,according to the target combustion mode set in the step S13 (step S14).As mentioned above, in this embodiment, as the torque reduction means,one of the reduction of the fuel injection amount to be injected fromthe injector 15 (second torque reduction step), and the ignitionretardation, i.e., retardation of the drive timing of the spark plug 16(first or third torque reduction step), is employed. An example ofcontrol of selecting one of the two means is as previously exemplifiedin the flowcharts of FIGS. 10(A) and 10(B). For example, when theexample illustrated in FIG. 10(B) is employed, a relationship betweenthe target combustion mode and the torque reduction means is as shown inthe following Table 1.

TABLE 1 Target Combustion Mode Torque Reduction Means SPCCI_ λ > 1Reduction of fuel injection amount (First air/fuel ratio mode) SPCCI_ λ= 1 Ignition retardation (Second air/fuel ratio mode) SI_ λ = 1 Ignitionretardation

Subsequently, the ECU 60 (determination part 63) operates to determinewhether or not there is a need for switching the SPCCI combustionbetween the first air/fuel ratio mode (λ>1) and the second air/fuelratio mode (λ=1) (step S15). Here, the determination about the switchingthe SPCCI combustion between the first air/fuel ratio mode (λ>1) and thesecond air/fuel ratio mode (λ=1) is performed based on the target basicengine torque set in the step S12, before the torque reduction amountfor the vehicle attitude control is subtracted therefrom.

The intervention of the determination step S15 is for the followingreason. When the SPCCI combustion is performed in the first air/fuelratio mode, the air/fuel ratio A/F is set to a lean A/F of about 25/1 to30/1, and, when the SPCCI combustion is performed in the second air/fuelratio mode, the air/fuel ratio A/F is set to be 14.7/1 (λ=1). During theprocess of performing the mode switching between the first air/fuelratio mode and the second air/fuel ratio mode, the engine comes into anunstable state in which the amount of intake air, the fuel injectionamount or the like to be supplied to each cylinder changes to allow theair/fuel ratio to make a transition to a value conforming to eachair/fuel ratio mode. In this state, if the torque reduction for thevehicle attitude control is overlappingly performed, a problem thatcombustion becomes unstable or misfire occurs is likely to arise. Inview of this, the determination section 63 operates to perform thetorque down for the vehicle attitude control when there is no need forthe mode switching (YES in the step S15), and to prohibit the torquedown for the vehicle attitude control when there is a need for the modeswitching (NO in the step S15). In the latter case, iso-torque modeswitching control of performing the mode switching without torquefluctuation (the after-mentioned control in FIG. 14 or 15) is executed.

When there is no need for the mode switching (YES in the step S15), theECU 60 (combustion control part 61) operates to set a target finalengine torque, from the target basic engine torque set in the step S12and the torque reduction amount set in the step S2 illustrated in FIG.11 (step #9 illustrated in FIG. 8) (step S16). This target final enginetorque is a torque obtained by subtracting the torque reduction amountfor the vehicle attitude control, from the required torque. Here, it isobvious that, when there is no request for execution of the vehicleattitude control, the torque reduction amount to be subtracted is zero.Then, the ECU 60 operates to set a target combustion pressure inside thecombustion chamber 6, based on the target final engine torque (stepS17).

Subsequently, the ECU 60 operates to set combustion process controltarget values, from the target combustion pressure set in the step S17and the target combustion mode set in the step S13 (step S18).Specifically, a target air amount to be supplied to the combustionchamber 6, a target self-ignition timing for developing the CIcombustion, a target SI rate, a target air/fuel ratio, a target ignitiontiming at which an air-fuel mixture is ignited by the spark plug 16,etc., are set as the control target values.

Here, the term “SI rate” means a rate of the heat release amount of theSI combustion to the entire heat release amount, in the SPCCIcombustion. Referring to FIG. 6, the inflection point X therein is atime point when the combustion type is switched from the SI combustionto the CI combustion. An area R1 of a part of the heat release ratewaveform located on the advance side with respect to a crank angle θcicorresponding to the inflection point X is defined as the heat releaseamount of the SI combustion, and an area R2 of the remaining part of theheat release rate waveform located on the retard side with respect tothe θci is defined as the heat release amount of the CI combustion. TheSI rate can be expressed using the areas R1, R2, as follows: SIrate=R1/(R1+R2).

FIG. 16 is a tabular diagram showing a relationship between the totalfuel injection amount and the ignition timing in each of various targetcombustion modes, in a case where the example illustrated in FIG. 10(B)is employed in the setting of the torque reduction means in the stepS14. In the first air/fuel ratio mode 71A (λ>1) of the SPCCI combustion,the second air/fuel ratio mode 72A (λ=1) of the SPCCI combustion, andthe SI combustion 73A, under the condition of “without torque reduction”for the vehicle attitude control, the total fuel injection amount is setto given values f1, f2, f3, respectively, and the ignition time is setto a given crank angle CA1.

On the other hand, in the first air/fuel ratio mode 71B (λ>1) of theSPCCI combustion under the condition of “with torque reduction” for thevehicle attitude control, the total fuel injection amount is changed toa value f4 which is reduced by a given amount with respect to the valuef1 in the first air/fuel ratio mode 71A under the condition of “withouttorque reduction”. Further, the target ignition timing is maintained atthe crank angle CA1, i.e., the ignition retardation is not performed.Further, in the second air/fuel ratio mode 72B (λ=1) of the SPCCIcombustion under the condition of “with torque reduction”, the totalfuel injection amount is maintained at the value f2, whereas theignition retardation is performed such that the target ignition timingis retarded from the crank angle CA1 to a crank angle CA2. Similarly, inthe SI combustion 73B under the condition of “with torque reduction”,the total fuel injection amount is maintained at the value f3, whereasthe ignition retardation is performed such that the target ignitiontiming is retarded from the crank angle CA1 to a crank angle CA2.

<Details of SPCCI Combustion Control>

FIG. 13 is a flowchart showing the details of the engine controlprocessing subroutine, and mainly showing steps relating to detailedcontrol of the SPCCI combustion. Following the step S18 in FIG. 12, theECU 60 operates to determine whether or not the SI rate is less than100%, i.e., whether or not the target combustion mode is the SPCCIcombustion (SI rate=100% means the SI combustion) (step S20).

When the target combustion mode is determined to be the SPCCI combustion(second combustion mode) (YES in the step S20), processing of settingcontrol values of actuators other than the injector 15 and the sparkplug 16 is first performed (steps S21 to S24). Specifically, the ECU 60(combustion control part 61) operates to set a target EGR rate, from thetarget air amount set in the step S18, and the in-cylinder temperatureassumed at the target self-ignition timing (step S21). In thisembodiment, EGR comprises internal EGR to be performed by control ofopening and closing timings of the intake valve 11 and the exhaust valve12 (see FIG. 2) (early opening of the intake valve 11 or late closing ofthe exhaust valve 12), and external EGR configured to recirculateexhaust gas to the intake passage via an EGR passage 51. Thus, in thestep S21, a target internal EGR rate and a target external EGR rate areset. Then, a target intake valve opening/closing timing and a targetexhaust valve opening/closing timing each of which is an opening/closingtiming of a respective one of the intake valve 11 and the exhaust valve12 for attaining the target internal EGR rate, and a target EGR valveopening which is the opening of the EGR valve 53 for attaining thetarget external EGR rate are set (step S22).

Subsequently, the ECU 60 operates to set, for attaining the target airamount, a target throttle opening which is the opening of the throttlevalve 32, a target bypass valve opening which is the opening of thebypass valve 39 of the bypass passage 38, and a target clutch engagementdegree which is the degree of engagement of the electromagnetic clutch34 of the supercharger 33 (step S23). Then, the ECU 60 operates totransmit operation instructions, respectively, to actuators of controltargets so as to achieve the target throttle opening, the target intakevalve opening/closing timing, the target exhaust valve opening/closingtiming, the target bypass valve opening, the target EGR valve openingand the target clutch engagement degree (step S24). That is, theactuators are operated according to the target values for achieving theSPCCI combustion, set in the step S18.

Subsequently, according to actual responsiveness of combustion withrespect to each of the target values, processing of correcting the fuelinjection amount and the fuel injection timing of the injector 15 andthe ignition timing of the spark plug 16 is performed (steps S25 toS29). A valve or the like configured to be driven by an actuator is adevice having relatively poor responsiveness, i.e., it is notimmediately moved just as the target value. An operation delay of such adevice exerts an influence on, e.g., attainment of the target air/fuelratio. The ECU 60 operates to figure out the degree of divergence of anactual combustion state with respect to a target combustion state, dueto the operation delay, and correct the fuel injection amount and thefuel injection timing of injector 15 having excellent responsiveness andthe ignition timing of the spark plug 16 having excellentresponsiveness, according to the state of internal gas actually formedin the combustion chamber 6, so as to correct the divergence.

Specifically, the ECU 60 operates to calculate the in-cylindertemperature, an intake charge amount and an in-cylinder oxygenconcentration in each cylinder 2 at an actual intake valve closingtiming (step S25). This calculation is performed by referring to: adetection value of the air flow sensor SN4; the state quantity of actualintake gas obtained from the first and second intake temperature sensorsSN5, SN7, the external EGR rate, etc.; the state quantity of actualinternal gas in each cylinder 2 obtained from detection values of theintake cam angle sensor SN12 and the exhaust cam angle sensor SN13,etc., and a combustion result in the last combustion cycle. As thecombustion result in the last combustion cycle, it is possible to usethe self-ignition timing obtained from the waveform of an actualin-cylinder pressure derived from a detection value of the in-cylinderpressure sensor SN3.

Subsequently, the ECU 60 operates to set, based on the intake chargeamount and the in-cylinder oxygen concentration calculated in the stepS25, a target fuel injection amount and a target fuel injection timingto attain the target air/fuel ratio set in the step S18 (step S26). Asexemplified in the charts (a) and (b) of FIG. 5, in the first air/fuelratio mode (λ>1) and the second air/fuel ratio mode (λ=1) of the SPCCIcombustion, the fuel injection is performed in a manner divided into twostages. Thus, the ECU 60 operates to determine the fuel injection amountand fuel injection timing for each of the 1st and 2nd fuel injections.Then, the ECU 60 operates to transmit an instruction to the injector 15to attain the target fuel injection amount and the target fuel injectiontiming (step S27).

Subsequently, the ECU 60 operates to correct the target ignition timingof the spark plug 16, based on the in-cylinder temperature of eachcylinder 2 at the actual intake valve closing timing (step S28).Specifically, the target ignition timing set in the step S18 iscorrected in a manner allowing the CI combustion to be started at thetarget self-ignition timing set in the step S18. Then, the ECU 60operates to drive the spark plug 16 to ignite an air-fuel mixture at thecorrected target ignition timing (step S29).

Referring to the step S20 again, when the SI rate is determined not tobe less than 100%, i.e., the target combustion mode is determined to bethe SI combustion (first combustion) (NO in the step S20), the ECU 60operates to set the target throttle opening, the target intake valveopening/closing timing, the target exhaust valve opening/closing timing,the target bypass valve opening, the target clutch engagement degree,the target EGR valve opening, etc., according to the target air amountset in the step S18 (step S30). Subsequently, the ECU 60 operates toset, based on the target air amount and the target combustion pressureset in the step S18, a target fuel injection amount and a target fuelinjection timing of the injector 15, and a corrected target ignitiontiming of the spark plug 16 (step S31). Then, the ECU 60 operates todrive the actuators, the injector 15 and the spark plug 16 to achievethe aforementioned target values (step S32).

<Mode Switching Control_Switching from λ=1 to Lean A/F>

Next, the iso-torque mode switching control (air/fuel ratio mode settingstep) to be executed when there is the need for switching the SPCCIcombustion between the first air/fuel ratio mode (λ>1) and the secondair/fuel ratio mode (λ=1), in the step S15, will be described. FIG. 14is a flowchart showing mode switching control in the case where there isthe need for switching from the second air/fuel ratio mode to the firstair/fuel ratio mode, and FIG. 17 illustrates time charts showingrelationships between the mode switching and respective ones of intakeair amount, fuel amount, ignition timing and air/fuel ratio.

When there is the need for the mode switching in the step S15illustrated in FIG. 12 (NO in the step S15), the processing subroutinetransitions to step S41 illustrated in FIG. 14. The ECU 60(determination part 63) operates to determine whether or not the modeswitching is switching from the second air/fuel ratio mode to the firstair/fuel ratio mode, i.e., switching of the SPCCI combustion from λ=1 tolean A/F (step S41). When the mode switching is determined to beswitching from the second air/fuel ratio mode to the first air/fuelratio mode (YES in the step S41), the determination part 63 operates toinstruct the combustion control part 61 to execute control of changingthe air/fuel ratio A/F from λ=1 to lean A/F, while maintaining theengine output torque constant during the mode switching.

Specifically, the ECU 60 (combustion control part 61) operates toincrease the intake air amount by adjusting the opening of the throttlevalve 32 (step S42), and increase the fuel injection amount of theinjector 15 (step S43). Referring to FIG. 17, a time period from time T0to time T1 is an execution period of the second air/fuel ratio mode, anda time period from the time T1 to time T2 is a mode switching periodfrom the second air/fuel ratio mode to the first air/fuel ratio mode.The ECU 60 operates to cause the intake air amount and the fuel amountin the time period from the time T0 to the time T1 to be proportionallyincreased in the time period between the time T1 and the time T2, asshown in the charts. Specifically, while the intake air amount isgradually increased to change the air/fuel ratio toward a lean side, thefuel amount is also gradually increased. This is intended to avoid asituation where an air-fuel mixture is formed to have an air/fuel ratiocausing production of NOx.

In parallel with the above, the ECU 60 operates to retard the ignitiontiming of the spark plug 16 in the time period between the time T1 andthe time T2 (step S44). This is intended to suppress a situation wherethe engine output torque is fluctuated toward an increase side, due toan increase in the fuel amount in the time period between the time T1and the time T2. The retardation of the ignition timing is performedsuch that, along with a gradual increase in the fuel amount, theignition timing is gradually shifted to a retard side. As a result ofthe ignition retardation, the engine output torque is reduced, so thatit is possible to cancel out an incremental torque corresponding to theincrease in the fuel amount, thereby maintaining the engine outputtorque constant in the time period between the time T1 and the time T2.

The ECU 60 operates to ascertain whether or not the intake air amounthas reached a target value of the intake air amount set for the firstair/fuel ratio mode (λ>1) (step S45). This target value of the intakeair amount is a value of the intake air amount capable of attaining anair/fuel ratio which is substantially free from producing NOx. In thisembodiment, an air/fuel ratio A/F of 25/1 is a rich limit of the firstair/fuel ratio (lean combustion) mode free from producing NOx, and anair/fuel ratio A/F of 30/1 is a given air/fuel ratio in the firstair/fuel ratio. Thus, in the step S45, it is determined whether theair/fuel ratio has reached 25. Then, when the air/fuel ratio isdetermined not to have reached 25 (NO in the step S45), the processingsin the steps S42 to S44 will be repeated. That is, the intake air amountand the fuel amount are further increased, and the ignition timing isfurther retarded.

On the other hand, when the intake air amount is determined to havereached a value capable of attaining an air/fuel ratio of 25 (YES in thestep S45), the ECU 60 operates to rapidly drop the fuel amount to avalue necessary for formation of a lean air-fuel mixture for the firstair/fuel ratio mode (step S46). The time T2 in the time charts of FIG.17 corresponds to the time point of the rapid dropping. As a result, anair-fuel mixture with an air/fuel ratio free from producing NOx in thefirst air/fuel ratio mode (λ>1) is formed in the combustion chamber 6.At this time point, the torque-down operation becomes unnecessary. Thus,the ECU 60 operates to terminate the ignition retardation (step S47).The intake air amount is successively increased even after the time T2.That is, the intake air amount is increased until time T2A when itreaches a value capable of attaining the given air/fuel ratio of 30.

<Mode Switching Control_Switching from Lean A/F to λ=1>

Next, with reference to FIGS. 15 and 17, the iso-torque mode switchingcontrol to be executed when there is the need for switching from thefirst air/fuel ratio mode (λ>1) to the second air/fuel ratio mode (λ=1)will be described. FIG. 15 is a flowchart showing mode switching controlin the case where there is the need for switching from the firstair/fuel ratio mode to the second air/fuel ratio mode,

When the mode switching is not switching from the second air/fuel ratiomode to the first air/fuel ratio mode (NO in the step S41), theprocessing subroutine transitions to step S51 illustrated in FIG. 15. Inthis case, the determination part 63 of the ECU 60 operates to instructthe combustion control part 61 to execute control of changing theair/fuel ratio A/F from lean A/F to λ=1, while maintaining the engineoutput torque constant during the mode switching.

Specifically, the ECU 60 (combustion control part 61) operates to reducethe intake air amount by adjusting the opening of the throttle valve 32(step S51). On the other hand, the fuel injection amount from theinjector 15 is maintained (step S52). Referring to FIG. 17, a timeperiod from the time T2 to time T3 is an execution period of the firstair/fuel ratio mode, and a time period from the time T3 to time T5 is amode switching period from the first air/fuel ratio mode to the secondair/fuel ratio mode. The ECU 60 operates to cause the intake air amountin the time period from the time T2A to the time T3 attaining the firstair/fuel ratio mode (λ>1) to be reduced in the time period between thetime T3 and the time T4, as shown in the charts. On the other hand, thefuel injection amount in the time period between the time T3 and thetime T4 is the same as that in the time period from the time T2A to thetime T3.

Subsequently, the ECU 60 operates to ascertain whether or not the intakeair amount has reached a given reduced intake air amount (air/fuelratio) (step S53). This reduced intake air amount is a value of theintake air amount capable of attaining an air/fuel ratio A/F of 25/1which a rich limit of the first air/fuel ratio (lean combustion) modefree from producing NOx. When the air/fuel ratio is ascertained not tohave reached 25 (NO in the step S53), the processing subroutine returnsto the step S51 in which the intake air amount is further reduced.

On the other hand, when the intake air amount is ascertained to havereached a value capable of attaining an air/fuel ratio of 25 (YES in thestep S53) at time T4, control of preventing production of NOx isexecuted. Specifically, the ECU 60 operates to gradually reduce theintake air amount (step S54), and, at the time T4, rapidly increase thefuel injection amount of the injector 15 to form an air-fuel mixturewith an air/fuel ratio of 14.7 (λ=1) based on a value of the intake airamount at the time T4 (step S55). In order to maintain λ=1, after thetime T4, the fuel injection amount is also reduced along with thereduction in the intake air amount. This makes it possible to avoid asituation where an air-fuel mixture is formed to have an air/fuel ratiocausing production of NOx. Further, the ECU 60 operates to, at the timeT4, rapidly retard the ignition timing of the spark plug 16 according tothe intake air amount and the fuel injection amount at the time T4, soas to cancel out an incremental torque corresponding to the increase inthe fuel amount (step S56), as with the aforementioned step S44. Thismakes it possible to prevent torque fluctuation around the time T4.

The ECU 60 operates to ascertain whether or not the intake air amounthas reached a target value of the intake air amount set for the secondair/fuel ratio mode (λ=1) (step S57). That is, it is ascertained whetheror not the intake air amount has been reduced to a value capable ofperforming the second air/fuel ratio mode, although the air/fuel ratiois reduced to 14.7 at the time T4. When the intake air amount isascertained not to have been reduced to the value (NO in the step S57),the processings in the steps S54 to S56 will be repeated. That is, theintake air amount and the fuel amount are further increased, and theignition timing is gradually recovered. This makes it possible tomaintain the output torque constant in the time period from the time T4to the time T5.

On the other hand, the intake air amount is ascertained to have reachedthe target value for the second air/fuel ratio mode (λ=1) (YES in thestep S57), the ECU 60 operates to stop further reducing the intake airamount and the fuel injection amount (step S58). The time T5 in the timecharts of FIG. 17 corresponds to the time point of the stopping. In thisway, an air-fuel mixture with λ=1 satisfying the intake air amount forthe second air/fuel ratio mode is formed in the combustion chamber 6.Then, the ECU 60 operates to terminate the ignition retardation at thetime T5 (step S59). At a time point just before the time T5, the torquereduction by the ignition retardation is automatically minimized. Aftercompletion of the step S47 in FIG. 14 or the step S59 in FIG. 59, theprocessing subroutine transitions to “RETURN” in FIG. 13, i.e., returnsto the step S11 in FIG. 12, and the same processings will be repeated.

[Modifications]

Although the present invention has been described based on oneembodiment thereof, it is to be understood that the present invention isnot limited to the embodiment. For example, the embodiment may bemodified as follows.

(1) As an example of a vehicle, the above embodiment shows the vehicle100 composed of a front-engine, front-wheel drive vehicle. However, theengine control method and the engine system of the present invention canalso be applied to a front-engine, rear-wheel drive vehicle, afour-wheel-drive vehicle, and a hybrid vehicle using, as a drivingsource, a motor configured to be driven by electric power supplied froma battery or a capacitor, and an internal combustion engine.(2) The above embodiment shows an example where the torque reduction forthe vehicle attitude control is prohibited when there is the need forthe mode switching between the first air/fuel ratio mode (λ>1) and thesecond air/fuel ratio mode (λ=1) (NO in the step S15 illustrated in FIG.12). However, in a situation where combustion is less likely to becomeunstable because the air/fuel ratio of an air-fuel mixture formed in thefirst air/fuel ratio mode is close to λ=1, the determination in the stepS15 may be omitted to allow the torque reduction for the vehicleattitude control to be always performed.(3) Further, when there is the need for the mode switching, instead ofprohibiting the torque reduction for the vehicle attitude control, themode switching may be temporarily prohibited during execution of thevehicle attitude control.(4) The above embodiment shows one example in which, during the SPCCIcombustion (example in FIG. 10(A)) or during the first air/fuel ratiomode of the SPCCI combustion (examples in FIG. 10(B) and FIGS. 12 to15), means to attain the torque reduction for the vehicle attitudecontrol is completely switched from the ignition retardation(retardation control) to the fuel amount reduction control.Alternatively, a part of the torque reduction for the vehicle attitudecontrol may be attained by the ignition retardation (control of limitingthe degree of the ignition retardation), and the remaining part may beattained by the fuel amount reduction control.

LIST OF REFERENCE SIGNS

-   1: engine-   2: cylinder-   15: injector (fuel injector)-   16: spark plug-   60: ECU (controller)-   61: combustion control part-   62: vehicle attitude control part-   63: determination part-   100: vehicle-   120: front road wheel (steerable road wheel/drive road wheel)-   107: accelerator pedal-   SN10: accelerator position sensor (operating state sensor)-   SN11: steering angle sensor

The invention claimed is:
 1. A control method for an engine which ismounted to a vehicle having steerable road wheels and mechanicallycoupled to drive road wheels of the vehicle, and which includes a sparkplug and a fuel injector, the control method comprising: a combustionmode setting step of selecting a combustion mode of the engine between afirst combustion mode in which an entirety of an air-fuel mixture in acylinder of the engine is combusted by a propagation of a flame producedby the spark plug, and a second combustion mode in which at least a partof an air-fuel mixture in the cylinder is combusted by a self-ignition,on the basis of an operating state of the engine; a decremental torquesetting step of setting a torque reduction amount by which an outputtorque of the engine is to be reduced, on the basis of a steering angleof the steerable road wheels; a first torque reduction step ofcontrolling the spark plug based on the torque reduction amount set inthe decremental torque setting step so as to retard an ignition timing,when the first combustion mode is selected in the combustion modesetting step; and a second torque reduction step of controlling the fuelinjector based on the torque reduction amount set in the decrementaltorque setting step so as to reduce a fuel injection amount, when thesecond combustion mode is selected in the combustion mode setting step.2. The control method according to claim 1, further comprising anair/fuel ratio mode setting step of, when the second combustion mode isselected in the combustion mode setting step, selecting an air/fuelratio mode between a first air/fuel ratio mode in which the air-fuelmixture is set to be leaner than a stoichiometric air/fuel ratio, and asecond air/fuel ratio mode in which the air-fuel mixture is set to beequal to or richer than the stoichiometric air/fuel ratio, on the basisof the operating state of the engine, wherein the second torquereduction step includes controlling the fuel injector based on thetorque reduction amount set in the decremental torque setting step so asto reduce the fuel injection amount, when the first air/fuel ratio modeis selected in the air/fuel ratio mode setting step.
 3. The controlmethod according to claim 1, further comprising: an air/fuel ratio modesetting step of, when the second combustion mode is selected in thecombustion mode setting step, selecting an air/fuel ratio mode between afirst air/fuel ratio mode in which the air-fuel mixture is set to beleaner than a stoichiometric air/fuel ratio, and a second air/fuel ratiomode in which the air-fuel mixture is set to be equal to or richer thanthe stoichiometric air/fuel ratio, on the basis of the operating stateof the engine; and a third torque reduction step of, when the secondair/fuel ratio mode is selected in the air/fuel ratio mode setting step,controlling the spark plug based on the torque reduction amount set inthe decremental torque setting step so as to retard the ignition timing.4. The control method according to claim 2, further comprising: anair/fuel ratio mode setting step of, when the second combustion mode isselected in the combustion mode setting step, selecting an air/fuelratio mode between a first air/fuel ratio mode in which the air-fuelmixture is set to be leaner than a stoichiometric air/fuel ratio, and asecond air/fuel ratio mode in which the air-fuel mixture is set to beequal to or richer than the stoichiometric air/fuel ratio, on the basisof the operating state of the engine; and a third torque reduction stepof, when the second air/fuel ratio mode is selected in the air/fuelratio mode setting step, controlling the spark plug based on the torquereduction amount set in the decremental torque setting step so as toretard the ignition timing.
 5. An engine system, comprising: an enginemounted to a vehicle having steerable road wheels and mechanicallycoupled to drive road wheels of the vehicle, the engine including aspark plug and a fuel injector; an operating state sensor configured todetect an operating state of the engine; a steering angle sensorconfigured to detect a steering angle of the steerable road wheels; anda controller, wherein the controller is configured to: select acombustion mode of the engine between a first combustion mode in whichan entirety of an air-fuel mixture in a cylinder of the engine iscombusted by a propagation of a flame produced by the spark plug, and asecond combustion mode in which at least a part of an air-fuel mixturein the cylinder is combusted by a self-ignition, on the basis of adetection result by the operating state sensor; set a torque reductionamount by which an output torque of the engine is to be reduced, on thebasis of a detection result by the steering angle sensor; control thespark plug based on the set torque reduction amount so as to retard anignition timing, when the first combustion mode is selected as thecombustion mode of the engine; and control the fuel injector based onthe set torque reduction amount so as to reduce a fuel injection amount,when the second combustion mode is selected as the combustion mode ofthe engine.